Investigation of boundary layer behaviour in HCCI combustion using chemiluminescence imaging

A five-cylinder diesel engine, converted to a single cylinder operated optical engine is run in Homogeneous Charge Compression Ignition (HCCI) mode. A blend of iso-octane and n-heptane is used as fuel. An experimental study of the horizontal boundary layer between the main combustion and the non-reacting surface of the combustion chamber is conducted as a function of speed, load, swirl and injection strategy. The combustion behaviour is monitored by chemiluminescence measurements. For all cases an interval from -10 to 16 crank angles after top dead center (CAD ATDC) in steps of one CAD are studied. One image-intensified camera observes the boundary layer up close from the side through a quartz cylinder liner while a second camera has a more global view from below to see more large scale structure of the combustion. The averaged chemiluminescence intensity from the HCCI combustion is seen to scale well with the rate of heat release. A boundary layer is defined and studied in detail between the main combustion volume and the piston crown surface as a function of crank angle. The boundary layer is found to be in the range from 2 to 4 mm for all cases by the definition used; however, the location for the measurements becomes more and more important as combustion becomes more inhomogeneous. To get accurate calculations, the level of noise must also be considered and definitions of boundary layer thickness should not be made at to low chemiluminescence intensity.